This invention pertains to a filter assembly for filtering coolant air that passes through an avionic line replaceable unit (LRU) box.
In aircraft, electronic and electrical circuitry is customarily retained in LRUs which are essentially boxes that have air inlet and outlet holes on their top and bottom surfaces. The LRUs are inserted in trays whose dimensions and configurations are standardized in accordance with the requirements of the avionic industry. Multiple circuit electrical connectors are typically mounted in the rear part of the tray and mating electrical connectors are mounted on the rear of the LRUs so that when the LRU is slid into the tray, the mating connectors join and a plurality of electrical circuits are completed. The bottom or base of the tray is generally planar and has a large opening, typically, for communicating the air passing into or out of the LRU with a large plenum whose interior may be at a positive pressure relative to atmospheric pressure or, in some installations, the pressure may be negative relative to atmospheric pressure. In any case, an opening in the base of the tray will have a gasket that effects a seal with the bottom of the LRU box and with the underlying plenum. Moreover, the opening in the plenum is typically subdivided into a plurality of openings which can be selectively plugged to accomplish what is called metering of the airflow through the LRU box. In dedicated designs no holes are plugged because the number of holes are selected so their total cross section provides the air flow required n a particular LRU.
In existing aircraft, the customary practice is to maintain the plenum or shelf interior at a negative pressure such that air is drawn in through the top holes of the LRU box and is drawn out through holes in the bottom of the box for being conducted away and possibly discharged to the atmosphere through a conduit in which there is a suction or pressurizing fan that maintains the desired negative pressure condition in the plenum.
The LRUs are mounted in various compartments in the aircraft such as in a compartment beneath the pilot's cabin or in a compartment located above or below the passenger cabin, either into the compartment or elsewhere. The cooling air for the LRUs is drawn into such compartment by various means, including ventilation systems, ventilation ports, doors, hatches and the like and discharged either into the compartment or elsewhere by way of the plenum or shelf air conduit. Trays may be installed in compartments which are in air circulation communication with the passenger compartment, the baggage compartment and other sub-cabin compartments wherein the air may contain various contaminants such as smoke, grease, aerosols, flies, insects, hair and other organic matter in aircraft which carry live animals, for example. These contaminants are entrained in the coolant air that is either forced through or drawn through the LRUs. The result is that electronic components in the LRUs, such as circuit boards or circuit elements, acquire a coating of contaminants which, in some cases, are conductive and in any case constitute a barrier for heat transmission. It is well known that the operating life of electronic circuits is correlated to the temperature at which they operate When dirty or contaminated air is circulated through the LRU boxes, failures of the circuitry occur prematurely, and such failure, needless to say, may jeopardize the safety of the aircraft and its passengers, as well as be very costly for the aircraft operator.
It is also important to aircraft operators that replacing an LRU involves a more demanding procedure than simply unplugging one unit and inserting a substitute unit into the avionic tray. If the connection between the multi pin plug and socket connectors on the LRU and on the avionic tray are separated, it is required in compliance with governmental and avionic maintenance industry rules to check the integrity and proper function of every one of the plurality of circuits that are made by the multiple line connectors before the replacement process can be accepted, and the aircraft can be restored to operation. It will be evident, therefore, that it would be highly desirable to have a coolant air filter assembly associated with each LRU that not only provides for filtering the input air to the LRU and assures uniform airflow through the LRU but also allows replacement of the dirty or contaminated filter itself without requiring that the LRU be disconnected, withdrawn from the avionic tray or otherwise disturbed